The present invention relates generally to electrical discharge machining (EDM), and, more specifically, to EDM drilling.
Electrical discharge machining is a process in which a cathodic electrode is positioned atop an electrically conducting workpiece and a liquid dielectric is channeled therebetween. Electrical current passes between the electrode and workpiece and locally erodes the workpiece for machining thereof. In a typical application, the electrode may be used for drilling a hole of any desired shape in the workpiece.
For example, many gas turbine engine components include small holes therein through which cooling air is channeled during operation. The holes are small in diameter and typically range from 10-80 mils (0.2-2.0 mm), and require a slightly smaller diameter EDM electrode.
The narrow electrodes are consumed during machining, and are therefore initially relatively long in length which typically ranges from about 12-16 inches (30-41 cm) for obtaining a useful life during drilling. Furthermore, the electrodes are typically tubular for channeling the liquid dielectric therethrough during operation. Accordingly, the hollow, slender electrodes are relatively flexible in bending along their longitudinal axes. Such flexibility is typically not desirable since it adversely affects the accuracy and repeatability of EDM drilling.
More specifically, the electrode tip must be accurately maintained at a small clearance gap of about 1 mil (0.25 mm) with the workpiece to effect suitable electrical discharge machining without experiencing an undesirable electrical short circuit therewith. Accordingly, the electrode tip is typically mounted through a lower guide which accurately maintains a side clearance around the electrode as it drills through the workpiece. And, the opposite or top end of the electrode is held in a conventional electrode holder which is effective for translating the electrode downwardly toward the workpiece during operation, and for maintaining the small clearance gap vertically therebetween. In this way, the tip guide and electrode holder accurately support both ends of the electrode for maintaining the desired gap both laterally around the electrode tip and vertically between the tip and the workpiece during the EDM operation.
However, the electrodes have a maximum length at the beginning of the drilling operation, with corresponding maximum flexibility, and are consumed during drilling which decreases their length and flexibility correspondingly.
Electrode flexibility becomes a problem due to the substantially high pressure of the dielectric channeled therethrough. Dielectric pressures up to about 50 atmospheres are conventional and produce a jet of dielectric discharge from the electrode tip against the workpiece as a hole is drilled. The electrode correspondingly experiences a reaction force which acts in compression therethrough. Since the electrode is a slender rod or column, it is subject to compressive buckling loads which can cause lateral deflection of electrode which correspondingly shortens its effective length and withdraws the electrode tip slightly away from the workpiece. In response thereto, the electrical controller which feeds the electrode toward the workpiece effects additional feed. However, the reaction flexing of the electrode due to the dielectric discharge is a periodic phenomena which causes the controller to correspondingly retract and feed the electrode at the corresponding frequency. This transient effect adversely affects the ability to maintain accurate depth control of the electrode.
A conventional solution to this problem includes the use of precision ground tubular brass electrodes. This is a relatively expensive process to impart residual stress in the ground electrodes which increases their rigidity and therefore decreases the undesirable flexing thereof during EDM drilling.
Accordingly, it is desired to improve EDM drilling for reducing electrode flexing and decreasing the cost of electrodes therefor.